The major objective of the proposed research is to determine by X-ray crystallographic technique the three-dimensional structure of a leucine, isoleucine, valine-binding protein (MW equals 36,000), an essential component of the high-affinity transport system for these branched aliphatic amino acids in Escherichia coli K12. The same technique will be utilized to determine the stereochemistry of binding of these amino acids to the protein. Crystals of the leucine, isoleucine, valine-binding protein that are suitable for high-resolution structural analysis have been obtained. The crystals belong to the space group P2 sub1 2 sub1 2 sub1, unit cell dimensions: 40.1 x 71.2 x 115.9 A. The unit cell contains four binding-protein molecules. Two methods will be utilized to determine the phases of the leucine, isoleucine, valine-binding protein; namely, molecular replacement (using structure information from L-arabinose-binding protein) and multiple isomorphous replacement. The determination of the complete amino acid sequence of the protein has been reported by other investigators.